The majority of electronic circuitry assembled today utilizes surface mounted components, soldered onto the surface of printed circuit boards. The solder is applied to the printed circuit boards by screen printing or stencil printing solder paste onto the desired locations, placing the surface mount components into the solder paste, and subsequently thermally fusing the solder paste with an oven, infrared heater or convection heater.
The conventional state-of-the-art solder paste consists of small particles of solder suspended in a screening vehicle that contains volatile solvents and flux. The requirements and specifications governing solder pastes and soldering flux for electronics use are cited in the American National Standards J-STD-004, and J-STD-005, jointly issued in January 1995 by the Electronic Industries Association (EIA), 2001 Pennsylvania Ave., NW., Washington D.C. 20006 and the Institute for Interconnecting and Packaging Electronic Circuits (IPC), 7380 N. Lincoln Ave, Lincolnwood, Ill. 60646. The specifications and requirements for the solder alloy compositions and solder powders used in electronic grade solder pastes are cited in American National Standard J-STD-006. The solder composition in the spherical powders used to make up solder pastes is usually of a homogeneous pre-fused alloy composition specific to the desired melting point of the solder and its final intended use or application. Solder powders and solder pastes of various melting points and alloy compositions are readily available from a number of foundry sources, including: Alpha Metals, Jersey City, N.J. 07304; Advanced Metals Technology Inc., Branford, Conn. 06405; or Indium Corporation of America, Utica, N.Y. 13502.
The known art also contains the technology to coat copper and silver particles with low melting solders (U.S. Pat. No. 4,097,266, incorporated herein by reference), and the use of such coated copper spheres in solder paste as standoffs for components on electronic circuits (U.S. Pat. No. 4,731,130 incorporated herein by reference). All of these solder powders and solder pastes are fused in their final use application by heating with an external heating source, such as an oven or soldering iron. One problem with this technology is that sophisticated ovens are now required to closely control the temperature of the soldering operation in order to create the precisely structured miniature solder joints commonly found in today's electronic assemblies. The soldering process has become very demanding and often cannot be properly performed because some components mask the heat source from the solder joint, thus causing poor reflow of the solder. In other cases, the large mass of the part relative to the amount of solder causes unequal heating of the solder, again resulting in a poor solder joint. What is needed is a solder powder, solder paste, or soldering system that does not need direct physical contact with thermal heat sources such as ovens or solder irons to effect fusion or melting of the solder.